Apertured films, woven fabrics and nonwoven materials have been used as filter sheets for removing or separating particles from liquids. Generally speaking, such filter sheets rely on some form of mechanical straining or physical entrapment. Such filter sheets can pose limitations when the size of the particle to be removed is small relative to the average pore diameter of the filter sheet (e.g., large pressure drops across the filter sheet). For nonwoven materials, this is particularly true for particles of less than one micron in diameter.
Improved filters have been developed with modified surface charge characteristics to capture and adsorb particles by electrokinetic interaction between the filter surface and particles contained in an aqueous liquid.
It would be desirable if certain inexpensive materials could be used to produce filters having modified surface charge characteristics for filtering very fine particles from aqueous liquids. For example, polyolefins are widely used in the manufacture of sheets of apertured films, woven fabrics, and nonwoven materials. Many types of polyolefin sheets tend to be hydrophobic and relatively inert. That is, the low surface free energy of polyolefins (e.g., polypropylene) and their relatively chemically inert nature render many unmodified polyolefins ill-suited for filter sheet applications in which a modified surface charge is desired for removing particles from an aqueous liquid.
For example, many chemical charge modifiers (e.g., cationic resins, charged particles, etc.) adhere poorly, if at all, to conventional unmodified, hydrophobic polyolefin sheets.
In the past, chemical coatings and/or internal additives have been added to filter sheets made of inexpensive materials to impart desired properties. Many of these coatings and/or additives present problems related to cost, effectiveness, durability and/or the environment.
It has been proposed that biofunctional materials (e.g., proteins) can be deposited from solutions onto different substrates (i.e., sheets of materials) to modify the surface properties of the substrates and/or serve as a functionalized surface that can be chemically reactive. However, many of the economically desirable substrates (e.g., substrates formed of polymers such as polyolefins) have surfaces that are unsuitable for the rapid and inexpensive deposition of biofunctional materials, especially when durable, tightly-bound coatings of satisfactory adherence are desired.
Even if inexpensive, durable, tenacious coatings could be adhered to an economically desirable substrate, coatings composed solely of biofunctional materials (e.g., proteins) may have limitations, especially if the coatings lacked the desired chemical characteristics such as, for example, modified surface charge characteristics.
Thus, there is still a need for a practical and inexpensive chemically charged modified filter for removing charged, micron to sub-micron sized particles from an aqueous liquid. A need exists for such a filter formed from an unmodified, relatively inert, hydrophobic substrate such as, for example, an unmodified, relatively inert, polyolefin substrate. A need also exists for a method of removing charged, micron to sub-micron sized particles from aqueous liquid utilizing a practical and inexpensive chemically charge modified filter such as, for example, a chemically charge-modified filter formed from an unmodified, relatively inert, polyolefin substrate.
In addition to the needs described above, there is a need for a simple, practical ard inexpensive chemically charge-modified filter for removing waterborne pathogens from aqueous liquid. This need also extends to a simple method for removing waterborne pathogens from aqueous liquid utilizing a practical and inexpensive chemically charge-modified filter.
One phenomena observed with some filters having modified surface charge characteristics is that the filters have different filtration efficiencies for different types of waterborne pathogens, such as, for example, different types of bacteria. That is, some filters having modified surface charges provide acceptable removal of some types of waterborne pathogens (e.g., some types of bacteria) but not others. The nature of this affinity appears to be difficult to predict. Since even relatively small differences in waterborne pathogen removal efficiency can be important, the discovery that a filter or filter system has an unpredictably strong affinity for a waterborne pathogen would be both unexpected and highly desirable, especially if the filter can be used to produce potable water. Meeting this reed is important because removing waterborne pathogens from aqueous liquids in a practical and inexpensive manner remains a challenge in many parts of the world.
Definitions
As used herein, the term "chemical charge modifier" refers to polyvalent cations, polyvalent anions, positively charged particles and/or negatively charged particles. Exemplary chemical charge modifiers may have a positive charge and, when present in a liquid having a dielectric constant sufficient for separate charged particles to exist, can be incorporated or adsorbed onto a negatively charged amphiphilic macromolecule (e.g., amphiphilic protein) coating on a substrate to modify the coating so that cationic species and/or positively charged particles are present at the surface of the coating under the appropriate conditions. Exemplary chemical charge modifiers include, but are not limited to, Ca.sup.2+, Al.sup.3+, Bi.sup.3+ ions, and AlO(OH), Bi(OH).sub.3, iron/magnesium(Fe/Mg) particles, cationic polymers such as, for example, quaternary amine containing cationic resins, and aliphatic amines having at least one primary amine and/or secondary amine, and the like. Generally speaking, when the chemical charge modifier/amphiphilic macromolecule (e.g., amphiphilic protein) coated substrate is in contact with aqueous liquid having a pH above the isoelectric point of the amphiphilic macromolecule (e.g., amphiphilic protein) component, cationic species and/or positively charged particles may be adsorbed via electrostatic or electrokinetic interactions between the negatively charged amphiphilic macromolecule coating and positively charged chemical charge modifier.
As used herein, the term "chemically charge-modified" refers to the incorporation of chemical charge modifiers (e.g., polyvalent cations, positively charged particles, cationic polymers and aliphatic amines having at least one primary amine and/or secondary amine, and the like) onto an amphiphilic macromolecule (e.g., amphiphilic protein) coating deposited on a relatively inert, hydrophobic substrate such as, for example, a polyolefin substrate. Generally speaking, charge modification occurs when the chemical charge modifier/amphiphilic macromolecule (e.g., amphiphilic protein) coated substrate is in contact with aqueous liquid having a pH above the isoelectric point of the amphiphilic macromolecule (e.g., amphiphilic protein) component.
As used herein, the term "waterborne pathogens" refers to microorganisms existing in water or aqueous liquids that are capable of causing disease. Exemplary waterborne pathogens include, but are not limited to, Vibrio cholerae, Giardia lamblia, and Cryptosporidium parvum. Waterborne pathogens may range in size from about 0.02 micron to more than about 0.1 micron and includes the class of pathogens commonly referred to as "viruses." Many types of waterborne pathogens may be greater than about 0.1 micron in size.
As used herein, the term "amphiphilic protein" refers to proteins having both hydrophobic regions and hydrophilic regions. For example, amphiphilic proteins may be selected from classes of globular and/or random coil proteins. As another example, amphiphilic proteins may be milk proteins. As a further example, amphiphilic proteins may include proteins such as those found in bovine milk including, but not limited to, various caseins and whey proteins.
As used herein, the term "relatively low surface energy" refers to surface energies (i.e., surface free energies) attributed to materials that are not generally considered to be water wettable. Generally speaking, such materials have a surface energy of less than about 45 dynes per centimeter (dynes/cm) as determined in accordance with critical surface tension of wetting techniques described by Bennet, M. K. and Zisman, W. A.; Relation of Wettability by Aqueous Solutions to the Surface Constitution of Low Energy Solids; J. Phys. Chem., pps. 1241-1246, Volume 63 (1959). Many such materials have a surface energy of ranging from about 29 to about 35 dynes/cm.
As used herein, the term "relatively high surface tension" refers to a level of attractive force in a liquid exerted by the molecules below the surface upon those at the surface/air interface, resulting from the high molecular concentration of a liquid compared to the low molecular concentration of a gas. Relatively high surface tensions are characteristic of, for example, some aqueous liquids and/or aqueous solutions having little or no added surfactants or other agents that reduce the surface tension. Surface tension may be determined from measurements of the contact angle of sessile drops using a goniometer such as, for example goniometer model No. 100-00 115 (equipped with videocamera) available from Rame-Hart, Inc., or by methods such as, for example, DuNouy ring methods. Relatively high surface tension for the purposes of the present invention is a surface tension of at least about 45 dynes/cm. Desirably, the surface tension is greater than 45 dynes/cm.
As used herein, the term "shear stress conditions" refers to conditions under which a shearing stress (force per unit area) is applied to a liquid. As an example, for a given volume of a liquid, increasing the rate at which the liquid penetrates or passes through a relatively permeable sheet such as, for example, a polyolefin nonwoven fibrous web (i.e., by decreasing the exposure time) results in an increased shear stress at the fiber/liquid interface. In this case, a long exposure time generally indicates little or no shear stresses and a short exposure time generally indicates shear stress conditions. Shear stress conditions may occur in liquid flow having generally laminar or turbulent flow characteristics.
As used herein, the term "adsorbed" refers to a type of adhesion which takes place at the surface of a solid in contact with another medium (e.g., a liquid), resulting in the accumulation or increased concentration of particles, waterborne pathogens and/or molecules from that medium in the immediate vicinity of the surface.
As used herein, the term "nonwoven web" refers to a web that has a structure of individual fibers or filaments which are interlaid, but not in an identifiable repeating manner. Nonwoven webs have been, in the past, formed by a variety of processes known to those skilled in the art such as, for example, meltblowing, spunbonding, wet-forming and various bonded carded web processes.
As used herein, the term "spunbonded web" refers to a web of small diameter fibers and/or filaments which are formed by extruding a molten thermoplastic material as filaments from a plurality of fine, usually circular, capillaries in a spinneret with the diameter of the extruded filaments then being rapidly reduced, for example, by non-eductive or eductive fluid-drawing or other well known spunbonding mechanisms. The production of spunbonded nonwoven webs is illustrated in patents such as Appel, et al., U.S. Pat. No. 4,340,563.
As used herein, the term "meltblown fibers" means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into a high-velocity gas (e.g. air) stream which attenuates the filaments of molten thermoplastic material to reduce their diameters, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high-velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. The meltblown process is well-known and is described in various patents and publications, including NRL Report 4364, "Manufacture of Super-Fine Organic Fibers" by V. A. Wendt, E. L. Boone, and C. D. Fluharty; NRL Report 5265, "An Improved Device for the Formation of Super-Fine Thermoplastic Fibers" by K. D. Lawrence, R. T. Lukas, and J. A. Young; and U.S. Pat. No. 3,849,241, issued Nov. 19, 1974, to Buntin, et al.
As used herein, the term "microfibers" means small diameter fibers having an average diameter not greater than about 100 microns, for example, having a diameter of from about 0.5 microns to about 50 microns, more specifically microfibers may also have an average diameter of from about 1 micron to about 20 microns. Microfibers having an average diameter of about 3 microns or less are commonly referred to as ultra-fine microfibers. A description of an exemplary process of making ultra-fine microfibers may be found in, for example, U.S. Pat. No. 5,213,881, entitled "A Nonwoven Web With Improved Barrier Properties".
As used herein, the term "apertured film-like material" refers to a generally flat or planar layer of material which has been punched, drilled, apertured, stretched, perforated, embossed, patterned, crinkled and/or otherwise processed so that it has relatively gross or visible openings with or without a pattern or texture in the thickness dimension (i.e., Z-direction) of the material. Exemplary apertured film-like materials include, but are not limited to, perf-embossed films, textured apertured films, reticulated apertured films, contoured apertured films, film-nonwoven apertured laminates, and expanded plexi-filamentary films.
As used herein, the term "sheet" refers to a material that can be a woven fabric, knit fabric, nonwoven fabric or film-like material (e.g., an apertured film-like material).
As used herein, the term "solution" refers to any relatively uniformly dispersed mixture of one or more substances (e.g., solute) in one or more other substances (e.g., solvent). Generally speaking, the solvent may be a liquid such as, for example, water and/or mixtures of liquids. The solvent may contain additives such as salts, acids, bases, viscosity modifiers, preservatives, disinfectants, anti-microbial agents and the like. The solute may be any material adapted to uniformly disperse in the solvent at the appropriate level, (e.g., ionic level, molecular level, colloidal particle level or as a suspended solid). For example, a solution may be a uniformly dispersed mixture of ions, of molecules, of colloidal particles, or may even include mechanical suspensions.
As used herein, the term, "permeable" and "permeability" refer to the ability of a fluid, such as, for example, a gas to pass through a particular porous material. Permeability may be expressed in units of volume per unit time per unit area, for example, (cubic feet per minute) per square foot of material (e.g., (ft.sup.3 /minute/ft.sup.2)). Permeability may be determined utilizing a Frazier Air Permeability Tester available from the Frazier Precision Instrument Company and measured in accordance with Federal Test Method 5450, Standard No. 191A, except that the sample size was 8".times.8" instead of 7".times.7". Although permeability is generally expressed as the ability of air or other gas to pass through a permeable sheet, sufficient levels of gas permeability may correspond to levels of liquid permeability to enable the practice of the present invention. For example, a sufficient level of gas permeability may allow an adequate level of liquid to pass through a permeable sheet with or without assistance of a driving force such as, for example, an applied vacuum or applied gas pressure. Generally speaking, a permeable sheet may have a permeability of at least about 20 cubic feet per minute per square foot (cfm/ft.sup.2), as measured for a substantially dry sheet prior to processing. It is contemplated that a sheet having a permeability of less than about 20 cfm/ft.sup.2, as measured for a substantially dry sheet prior to processing, could be used successfully in the practice of the present invention with (or in some cases without) assistance of a driving force such as, for example, an applied vacuum or applied gas pressure. As an example, a permeable sheet may have a permeability of from about 25 to over 200 cfm/ft.sup.2, as measured for a substantially dry sheet prior to processing. As another example, a permeable sheet may have a permeability of from about 35 to about 150 cfm/ft.sup.2, as measured for a substantially dry sheet prior to processing.
As used herein, the term "consisting essentially of" does not exclude the presence of additional materials which do not significantly affect the desired characteristics of a given composition or product. Exemplary materials of this sort would include, without limitation, pigments, antioxidants, stabilizers, surfactants, waxes, flow promoters, particulates or materials added to enhance processability of a composition.